The end of the Second World War sparked Hollywood’s golden age – red carpets, palm trees, Bogie and Bacall. But not far from all the glamour and glitz, a boy haunted the banks of the Los Angeles River, fishing for crawdads and hunting rabbits with a bow and arrow.

Although Yvon Chouinard lived in Burbank, his family roots were Québécois; as a boy, he had wanted to be a fur trapper. And although he would later make his mark as a hugely successful businessman, he grew up with a love of the wild and a passion for waterways that burns as intensely as ever nearly seven decades later.

Last month, that passion brought him back to Canada, where he and his film DamNation were the closing-night attraction at the Banff Mountain Film and Book Festival. The documentary, executive-produced by Mr. Chouinard, reveals something unsettling about many dams in the United States: They no longer serve any useful purpose.

The screening was sold out, largely because Mr. Chouinard’s love of the outdoor life also has brought him fame and great fortune. At 76, he is “America’s most unlikely business guru,” according to The Wall Street Journal, and his company – Patagonia, which is more successful than ever, with sales of about $600-million worth of high-end outdoor clothing this year – is “perhaps best understood as a sort of performance art.”

But this business guru is also well aware of plans that were given the official go-ahead this week to build a third massive hydroelectric dam on the Peace River in northeastern British Columbia. Known as Site C, it is expected to cost almost $9-billion – the biggest infrastructure investment the province has ever made – with construction to begin next summer and to take as long as a decade.

Mr. Chouinard questions B.C. Hydro’s case for flooding 55 square kilometres, much of it farmland and traditional First Nations territory. “It really is a fallacy that hydro is clean power,” he argues. “It’s like ‘clean’ coal. There’s no such thing. I mean, with wind turbines and solar, it’s pretty crazy to destroy an entire river, destroy an entire valley, destroy some of the best agricultural land in Canada.”